In downflow continuous casting it is standard practice to swirl or rotate the body of liquid metal about a vertical axis on which the mold is centered. Such rotation produces a very fine structure in the metal which is considered highly desirable, producing an extremely strong casting with a good surface.
Such rotation can be carried out either mechanically by means of a sample stirring instrument or pneumatically by means of a gas stream. More commonly, however, electromagnetic coils are arranged around the mold to inductively rotate it.
The main problem with this method is that the impurities which normally collect on the top of the molten metal, or even the slag intentionally left there, is partially sucked into the vortex formed at the center of the downwardly moving and swirling molten body. Thus the center of the casting thus produced will have a great deal of impure inclusions that considerably detract from the overall quality of the workpiece.
It has been suggested to eliminate this problem by injecting the melt into the mold below the surface thereof by means of a nozzle shaped like a funnel and having a lower outlet end that is positioned underneath the surface of the body of the melt, below the melt/slag interface. Such procedures do somewhat reduce the above-described problem, nonetheless the vortex which starts right at the top of the melt remains so that some of the material floating on top of the melt is, indeed, sucked down into the molten mass.